1. Field of the Invention
This invention relates to a fixing device and an image forming apparatus.
2. Description of Related Art
Fixing devices mounted on image forming apparatuses such as photocopiers, printers, facsimile machines employing electrophotographic methods, widely use thermally fixing methods. The fixing devices of the thermally fixing method generally have a fixing member and a pressure member forming a fixing nipping portion contacting each other with pressure and have a heating member incorporated in at least one of the fixing member and the pressure member. The fixing devices fixes unfixed toner images on a paper surface in application of heat and pressure when a paper carrying unfixed toner images passes the fixing nipping portion,
A known publication, such as Japanese Patent Application Publication (A1) 2003-15463, discloses as a fixing device of a thermal fixing method, a device including a fixing roller incorporating a heater, a pressure roller contacting the fixing roller with pressure, a thermistor detecting a surface temperature of the fixing roller, a control means for controlling the heater to be turned on and off according to the detected value of the thermistor, and a thermostat serving as an excessive temperature rise prevention device disposed in not contact with the fixing roller to make power supply to the heater shut down based on an extraordinary temperature where the temperature of the fixing roller rises in an extraordinary way.
Where the fixing device employs such an excessive temperature rise prevention device stopping heat application from a heat source when it is detected that the temperature of the member heated by the heat source reaches a prescribed temperature, it is required that the temperature of the member heated by the heat source is detected accurately.
It is therefore an object of the invention to provide a fixing device and an image forming apparatus capable of detecting accurately a temperature of a member heated by a heat source.